1. Field of the Invention
The present invention relates to a fuel injection control device for an internal combustion engine. More specifically, the invention relates to a fuel injection control device which is capable of attenuating the torsional vibration of the output shaft system of an internal combustion engine.
2. Description of the Related Art
A torsional vibration occurring in the output shaft system of an internal combustion engine causes various problems. Especially, in the engine for an automobile, the torsional vibration of the output shaft system causes a surge during a steady running as well as an acceleration shock during an acceleration of the vehicle and worsens the driveability of the vehicle. Further, in a diesel engine, since the fuel injection amount is determined in accordance with the operating condition of the engine (such as the engine speed and the amount of depression of the accelerator pedal by the driver), the determined fuel injection amount immediately changes when the amount of depression of the accelerator pedal (hereinafter referred to as the "accelerator depression" changes.
Therefore, if the amount of fuel determined by the accelerator depression is injected into the engine without correction, the torsional vibration of the output shaft system increases due to a sudden change in the output torque of the engine. Therefore, in order to prevent the increase in the torsional vibration, a so-called smoothing control of the fuel injection amount is performed to increase the actual fuel injection amount gradually even though the fuel injection amount determined by the accelerator depression suddenly increases during, for example, an acceleration of the vehicle.
Further, a method for attenuating torsional vibration by detecting the actual torsional vibration of the output shaft system of the engine and by controlling the engine output torque so that the detected vibration is attenuated is known in the art.
The example of a vibration attenuating device using this method is disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 60-26142.
The device in the '142 publication detects a physical amount relating to the torsional vibration such as a change in the rotating speed of a diesel engine, a change in the acceleration of the vehicle on which the engine is installed and a change in the torsional torque exerted on the output shaft of the engine and uses the value as parameters representing the characteristics of the torsional vibration of the engine output shaft system. The device in the '142 publication attenuates the torsional vibration by adjusting the amount of fuel injected into the engine by a feedback control based on a rate of change in the value of the parameter.
Theoretically, the device in the '142 publication can attenuate the torsional vibration of the output shaft system efficiently since it performs the feedback control of the actual fuel injection amount of the engine based on the actual torsional vibration. However, since the device performs the control based on the torsional vibration which has already occurred in the shaft system, the device cannot prevent the torsional vibration before it occurs. Therefore, device in the '142 publication cannot prevent a large torsional vibration occurs at the beginning of the transient condition such as the beginning of the acceleration of the vehicle.
A feedforward control can be also used for attenuating the torsional vibration. In feedforward control, for example, correction amounts for the fuel injection required for attenuating the torsional vibration are previously determined by experiments based on various combinations of the engine operating conditions and fuel injection amount calculated based on the engine operating conditions. The correction amounts obtained by experiment are stored in a memory of a control device and, during the operation of the engine, the correction amount of the fuel injection is read from the memory in accordance with the engine operating conditions and the calculated fuel injection amount. By correcting the calculated fuel injection amount using the correction amount determined by the feedforward control, the calculated fuel injection amount can be corrected before the actual torsional vibration occurs. Therefore, the torsional vibration does not occur even in the transient operating conditions. However, in order to perform an accurate feedforward control, it is necessary to determine the correction amounts based on every combination of the operating condition and calculated fuel injection amount for each type of the engine. Since this requires a large number of experiments, heretofore, it was difficult to apply feedforward control for accurately attenuating the torsional vibration.
Further, even if a feedforward control is applied, it is preferable to use the feedback control to compensate for the variation in the characteristics of the elements in the output shaft system and changes in the characteristics due to the accumulated operation time. If the feedback control is performed together with the feedforward control, however, the attenuation capability of the control sometimes deteriorates due to interference between the feedback control and the feedforward control.